An orange fat-staining dye (Toluene-azo napthol): (TAN) dye has been synthesized, tagged with radioactive iodine and injected into the aortas of normal and atherosclerotic rabbits. The dye is retained by the abnormal fat deposits in the aorta inner lining, and can be counted externally or outlined with X-ray film. However, the fat in normal aortas retains just as much total activity, probably because the non- diseased wall allows more complete penetration. Presently we are preparing colloid suspensions of the TAN in order to confine the reaction to the inner surface. It is hoped this will produce a test to detect early hardening of the arteries in humans.